Graft-versus-host disease (GVHD) is still a critical complication following allogeneic blood and marrow transplantation (BMT), most of which are performed between HLA-matched siblings. GVHD in these cases is due to differences in expression of minor histocompatibility antigens (miHA). In previous years of this project, we have clearly defined the relative etiological and pathological roles of both CD4+ and CD8+ T cell subsets in GVHD directed across miHA barriers in murinemodels. More recently, we have focused on the nature of these T cell subset responses in the miHA disparate B6->BALB.B (9.5 Gy) strain combination, by means of TCR Vb CDR3-size spectratype repertoire analysis, and we have found them to be somewhat heterogenous in Vb family involvement, yet limited in scope. Of most importance, we have been able to identify the Vb2 and 11 families of CD4+ T cells and the Vbl4 family of CD8+ T cells as capable of mediating severe GVHD by themselves, and at least in the former situation,their removal from transplanted inocula significantly diminishedGVHD potential. The general aim of this current proposal is to continue our investigation of the immunobiology of severe (lethal) GVHD by concentrating on the precise definition of the TCR specificities involved in disease pathogenesis, the mechanism by which specific Vb+ T cells mediate target cell injury, the localization and antigen expression requirements for T cell-mediated immunopathology, and the identification of the miHA that are responsible for the severity of these T cell responses. Our working hypothesis is that CD4+ and CD8+ T cell responses involved in the development of severe GVHD are driven by expression of "select" miHA in target organ tissue, and these miHA must be expressed by non-hematopoietictissue for immunopathological injury to occur. To test this hypotheses, we will focus our efforts on completely identifying the unique donor CD4+ and CD8+ T cell responses involved in the development of severe GVHD in the B6-> BALB.B (9.5 Gy) strain combination; identifying the host miHA target antigens responsible for severe immunopathology of GVHD, and determining the requirements for host miHA expression on hematopoietic and non-hematopoietic target tissue and the mechanism utilizedby CD4+ and CD8+ T cells to mediate target cell injury.